Thiopurine methyltransferase (TPMT, EC 2.1.1.67) is a cytoplasmic enzyme that catalyzes the S-adenosyl-L-methionine (Ado-Met)-dependent S-methylation of aromatic and heterocyclic sulfhydryl compounds. See L. C. Woodson et al., Biochem. Pharmacol., 32, 819 (1983); M. M. Ames et al., J. Med. Chem., 29, 354 (1986). Included among the substrates for TPMT are the thiopurine drugs, 6-mercaptopurine (6-MP) and azathioprine. See, C. N. Remy et al., J. Biol. Chem., 238, 1078 (1963). Mercaptopurine is an antimetabolite precursor that is used to treat a variety of leukemias, and as an immunosuppressive drug to treat arthritis, colitis and lupus. Azathioprine is a Class II immunosuppressive drug which is widely used in kidney transplantation.
The level of TPMT activity in human tissue is controlled by a common genetic polymorphism, and inherited variation in TPMT activity is an important factor responsible for individual differences in thiopurine drug toxicity and therapeutic efficacy. See, for example, P. R. Chocair et al., Transplantation, 53, 1051 (1992) and L. Lennerd, Lancet, 336, 225 (1990). As reported by R. M. Weinshilboum et al., Am. J. Human Genet., 32, 651 (1980), phenotypic expression of the TPMT genetic polymorphism can be determined in an easily accessible human cell, the red blood cell (RBC). This polymorphism also controls the level of enzyme activity in all other human tissues and cells that have been studied, including liver, kidney and lymphocytes. Patients with genetically low or undetectable RBC TPMT activity are at greatly increased risk for the development of life-threatening thiopurine-induced myelosuppression when treated with azathioprine or 6-MP, while patients with high TPMT activity may be undertreated with standard doses of these drugs, since they can so readily deactivate them.
Although the level of activity of TPMT and the TPMT-mediated metabolism of bioactive substances can be studied using experimental animals, a continuing need exists for a stable in vitro model system for mammalian TPMT activity that can be used to rapidly and reproducibly evaluate and predict the pathways of TPMT-mediated drug metabolism in humans.